The present invention relates to a method of and an apparatus for examining the alignment of beam axes of headlamps of an automobile on an assembly line.
The specification of Japanese Patent Application No. 64841/1981 (Japanese Patent Laid-Open No. 179639/1982) discloses a headlamp testing method and apparatus for examining the beam axes of automotive headlamps. In this known art, the light beam from a headlamp to be tested is applied to a screen, and the image of the light distribution pattern formed on the screen by the beam is taken up by a TV camera to produce video signals. The video signals are then quantized at a predetermined quantizing level so as to determine a closed curved area surrounded by a line indicative of illumination intensity higher than a predetermined level. This closed curved area will be referred to hereinunder as a "hot zone". The position of the geometrical center of the thus determined hot zone is regarded as being the position of beam axis of each headlamp. Marks indicating the allowable ranges of the beam axis alignment for different types of automobile are determined beforehand and the mark corresponding to the type of automobile to be examined is displayed on a monitor TV together with the position of the geometrical center of the hot zone. The operator then visually checks whether the position of the geometrical center of the hot zone falls within the mark which indicates the allowable range. If the geometrical center of the hot zone falls within the area of the mark representing the allowable range, the headlamp beam axis alignment is judged as being acceptable. If not, however, the headlamp beam axis is adjusted until the geometrical center of the hot zone comes to fall within the area of the pattern.
This known art, however, suffers from a disadvantage in that, since the picture signals of the light distribution pattern on the screen are quantized at a specific quantization level, even headlamps with the same specification provide different sizes and outlines of hot zone areas. This disadvantage is derived from a fluctuation in the light distribution pattern or a difference in the output voltage of the batteries mounted on the automobiles. Consequently, the accuracy of adjustment often fluctuates undesirably.
This problem will be explained in more detail with specific reference to FIGS. 1A to 1C and FIG. 2.
FIG. 1A is a chart which three-dimensionally shows the light distribution pattern of the headlamp beam applied to a screen. This chart is usually referred to as an "illumination intensity distribution chart". In this chart, X and Y axes represent coordinate values of the light distribution pattern on the screen, while LX axis shows the illumination intensity of the light distribution pattern. Symbols A1 to A5 denote, respectively, different closed curved areas corresponding to different illumination intensities which are represented by T.sub.H R1 ro T.sub.H R5. The differect closed curved areas A1-A5 are defined by lines indicating centain levels of illumination intensity, respectively. These closed curved areas A1 to A5 have respective geometrical centers which are represented by G1 to G5. A symbol MP represents the points of the maximum illumination intensities. FIG. 1B, shows a group of the closed curved areas A1 to A5 of different illumination intensity levels which are projected on the X-Y plane to overlap each other. Thus, FIG. 1B shows the light distribution pattern of the headlamp beam applied to the screen. If the video signals of the image of this light distribution pattern taken up by the TV camera are quantized at the illumination intensity level of T.sub.H R3, a hot zone HZ is obtained as shown in FIG. 1C. Obviously, this hot zone HZ corresponds to the closed curved area A3 of the illumination intensities higher than T.sub.H R3 shown in FIG. 1A. In the known method of examination of the headlamp beam axis, the position of the geometrical center G3 of this hot zone HZ is determined and displayed on the monitor TV as the position of the headlamp beam axis. As will be understood from FIG. 1A, the illumination intensity distribution exhibits a three-dimensional form similar to a mountain. This form, however, is not symmetrical with respect to the line MX indicative of the headlamp beam axis. In FIG. 1A, the line MX is difined by a line connecting the geometrical centers of respective closed curved areas A1 to A5. The degree of asymmetry is greater in the region where the illumination intensity is comparatively small than in the area in which the illumination intensity is comparatively large. Consequently, when the beam axis line, i.e., the positions of the geometrical centers of respective closed curved areas, are projected on the X-Y plane, the positions of these geometrical centers fluctuate over a wide area as shown in FIG. 2. In this figure, a symbol MP represent the point of the maximum illumination intensity. Symbols G1, G2 and G3 represents the positions of the headlamp beam axis determined in accordance with the quantizing levels of T.sub.H R1, T.sub.H R2 and T.sub.H R3. Symbols G3-1 and G3-2 show, respectively, the positions of the headlamp beam axis determined in accordance with the quantizing levels T.sub.H R3-1 and T.sub.H R3-2 which are intermediate between the illumination intensities T.sub.H R3 and T.sub.H R4. Similarly, a symbol G4 represents the position of the headlamp beam axis in accordance with T.sub.H R4, while G4-1 and G4-2 represent, respectively, the positions of the headlamp beam axis in accordance with T.sub.H R4-1 and T.sub.H R4-2 which are intermediate between the intensities T.sub.H R4 and T.sub.H R5. Symbols G5 and G5-1 show the positions of the headlamp beam axis in accordance with T.sub.H R5 and T.sub.H R5-1, respectively. The illumination intensity T.sub.H R5-1 is less than T.sub.H R5.
From FIG. 2, it will be understood that the desired accuracy of examination of the alignment of the headlamp beam axis is attained if the examination is made in the region in which the fluctuation of the beam axis position is comparatively small, e.g., within the region which contains the points MP, G1, G2 and G3. In other words, it will be understood that it is prefarable to examine the alignment of the headlamp beam axis by using, as the index of the alignment, the position of the geometrical center of the closed curved area which is comparatively close to the point of the maximum illumination intensity.
In the known method explained above, the quantization of the light distribution pattern is made at a constant intensity level of T.sub.H R3, for example. As stated above, even headlamps with the same specification often exhibit different patterns of illumination intensity distributions due to fluctuation of conditions such as the luminous intensity, light distribution pattern and the output voltage of the batteries mounted on the automobile. In such a case, the position of the beam axis may deviates widely from the expected beam axis position near the point of the maximum illumination intensity to the points G3-1 or G3-2 shown in FIG. 2, for example.
In order to obviate this problem, hitherto, a testing method has been proposed in which the maximum illumination intensity is determined for individual headlamps and the beam axis alignment is examined by using the position of the geometrical center of the hot zone surrounded by an illumination intensity level higher than 80% of the maximum illumination intensity. This method, however, cannot be practically carried out because an extremely complicated and troublesome process is required in order to specify the point of the maximum illumination intensity.